Artificial lifting systems for pumping downhole fluids such as crude oil or water, from a production well to the surface have been widely used in oil and gas industry. Existing artificial lifting systems include rod pumps, Electric Submersible Pumps (ESPs), Gas lift systems, Progressing Cavity Pumps (PCPs) and Hydraulic pumps.
Rod pumps generally comprises a sucker rod connecting to a subsurface pump, and a driver system coupled to the sucker rod for driving the sucker rod in a reciprocating motion for pumping downhole fluids to the surface. For example, traditional pumpjacks or horsehead pumps comprise a prime mover such as an electric motor or gas engine, which drives a set of gears to reduce the speed. The gears drive a pair of cranks, and the cranks in turn raise and lower one end of a beam having a “horse head” on the other end thereof. A steel cable, i.e., a bridle, connects the horse head to a downhole pump via a polished rod and sucker rods. The reciprocating up and down movement of the horse head then drives the downhole pump reciprocating between a fully retracted position and a fully extended position to pump the downhole fluid to the surface. The distance between the fully retracted position and the fully extended position is called a stroke. Generally, a stroke maybe a down-stroke that resets the rod pump downhole to the fully retracted position, or an up-stroke that moves the rod pump uphole to the fully extended position for pumping fluid to the surface.
Generally, long-strokes are preferable because, comparing to a rod pump with shorter pump stroke, a rod pump with longer pump stroke requires slower pumping speed for a given production rate, and therefore results in lower rod string stress and reduced power consumption.
The Sure Stroke Intelligent™ Lift System offered by Tundra Process solutions of Calgary, Alberta, Canada, the assignee of the subject patent application, uses a vertical hydraulic cylinder to drive a polished rod moving axially up and down, which in turn drives the downhole pump via sucker rods to pump downhole fluid to the surface with long strokes, e.g., ranging from 168 inches to 360 inches based on models.
U.S. Pat. No. 8,562,308, entitled “Regenerative Hydraulic Lift System”, to Krug, et al., discloses a hydraulic cylinder assembly for a fluid pump including a cylinder, a bearing attached to a about a first end of the cylinder, a rod slideably mounted within the bearing, and a piston located about an end of the rod in the cylinder opposite the bearing. A central axis of the rod is offset from, and parallel to, a centerline of the cylinder to impede a rotation of the piston about the rod. The hydraulic cylinder assembly further includes a hydraulic motor fluidly connected to the cylinder, the pump configured to provide a hydraulic pressure to the cylinder during an up-stroke of the piston and rod and the pump further configured to generate electricity on the down-stroke of the piston and rod.
U.S. Pat. No. 8,267,378, entitled “Triple Cylinder with Auxiliary Gas over Oil Accumulator”, to Rosman, discloses a hydraulic lift system for artificial lift pumping or industrial hoisting comprising a three chamber cylinder, a gas-over-oil accumulator, a large structural gas accumulator and a large flow pilot operated check valve. A matrix variable frequency drive, a standard variable frequency drive, an electrical squirrel cage motor or a natural gas engines are part of the main prime mover alternatives.
In above systems, a movable rod or plunger moves axially in a vertically oriented cylinder to drive the downhole rod pump for pumping fluid to the surface with long strokes. The stroke, however, may drift in operation due to change of environmental factors, such as change of temperature, downhole pump load, and the like. Large safety margins are usually applied to a top and bottom limit to such a stroke to avoid damage the cylinder and wellhead. Safety margins result in reduced stroke and reduced pumping effectiveness. Moreover, operators are thus required to regularly check the travel of the plunger, and reset top and bottom safety margins, causing burden to operators.
It is therefore an object to provide a novel method of automatically controlling an artificial lifting system to ensure its operation within a defined stroke range and an artificial lifting system employing same.